Thrust tectonics and Cretaceous intracontinental shortening in Southeast Alaska

An imbricate thrust belt extending along strike for more than 2000 km overprints the tectonic boundary between two of the largest allochthonous crustal fragments (Intermontane and Insular superterranes) in the North American Cordillera, and affects rocks west of the Coast Plutonic Complex in southeast Alaska and western British Columbia. Deformation was broadly coeval with mid-Cretaceous magmatism and involved the emplacement of west-directed thrust nappes over a structurally intact and relatively unmetamorphosed basement. The Palaeozoic and lower Mesozoic Alexander terrane forms structural basement for much of the thrust belt along a moderately northeast-dipping decollement. There were two main episodes of mid-Cretaceous deformation, which were contemporaneous with the emplacement of tabular plutonic bodies. Older structures record ductile southwest-vergent folding and faulting, and regional metamorphism, associated with a well-developed axial-planar foliation. Second-generation structures include southwest-directed thrust faults that juxtapose contrasting metamorphic grades and refold earlier structures. Structural, stratigraphical and geochronologic data suggest that regional-scale deformation in southeast Alaska occurred between 113 Ma and 89 Ma. Deformation involved the imbrication of marginal basin(s) and a magmatic arc, overprinting the older tectonic boundary between the Insular superterrane and the late Mesozoic western margin of North America (i.e. the Intermontane supertcrrane). Contractional deformation along the length of the thrust belt was broadly coeval with arc magmatism, and thus records intra-arc tectonism. Late Palaeocene to Early Eocene deformation and uplift may mark the transition from contractional to extensional tectonism, and perhaps records the collapse of tectonically thickened crust.